Which carbonyl carbon is more electrophilic

The lithium, sodium, boron and aluminum end up as soluble inorganic salts. The last reaction shows how an acetal derivative may be used to prevent reduction of a carbonyl function in this case a ketone.

Remember, with the exception of epoxides, ethers are generally unreactive with strong bases or nucleophiles. The acid catalyzed hydrolysis of the aluminum salts also effects the removal of the acetal.

This equation is typical in not being balanced i. If the saturated alcohol is the desired product, catalytic hydrogenation prior to or following the hydride reduction may be necessary. Before leaving this topic it should be noted that diborane, B 2 H 6 , a gas that was used in ether solution to prepare alkyl boranes from alkenes , also reduces many carbonyl groups.

Consequently, selective reactions with substrates having both functional groups may not be possible. In contrast to the metal hydride reagents, diborane is a relatively electrophilic reagent, as witnessed by its ability to reduce alkenes.

This difference also influences the rate of reduction observed for the two aldehydes shown below. The first, 2,2-dimethylpropanal, is less electrophilic than the second, which is activated by the electron withdrawing chlorine substituents. To see examples of these reactions, Click Here. The two most commonly used compounds of this kind are alkyl lithium reagents and Grignard reagents.

They are prepared from alkyl and aryl halides, as discussed earlier. These reagents are powerful nucleophiles and very strong bases pK a 's of saturated hydrocarbons range from 42 to 50 , so they bond readily to carbonyl carbon atoms, giving alkoxide salts of lithium or magnesium. Because of their ring strain, epoxides undergo many carbonyl-like reactions, as noted previously. Reactions of this kind are among the most important synthetic methods available to chemists, because they permit simple starting compounds to be joined to form more complex structures.

Examples are shown in the following diagram. A common pattern, shown in the shaded box at the top, is observed in all these reactions. The organometallic reagent is a source of a nucleophilic alkyl or aryl group colored blue , which bonds to the electrophilic carbon of the carbonyl group colored magenta.

The product of this addition is a metal alkoxide salt, and the alcohol product is generated by weak acid hydrolysis of the salt. The first two examples show that water soluble magnesium or lithium salts are also formed in the hydrolysis, but these are seldom listed among the products, as in the last four reactions.

Two additional examples of the addition of organometallic reagents to carbonyl compounds are informative. The first demonstrates that active metal derivatives of terminal alkynes function in the same fashion as alkyl lithium and Grignard reagents.

The second example again illustrates the use of acetal protective groups in reactions with powerful nucleophiles. Following acid-catalyzed hydrolysis of the acetal, the resulting 4-hydroxyaldehyde is in equilibrium with its cyclic hemiacetal. Reactions with Phosphorus and Sulfur Ylides The ylides are another class of nucleophilic organic reagents that add rapidly to the carbonyl function of aldehydes and ketones.

To learn about ylides and their reactions Click Here. The metal hydride reductions and organometallic additions to aldehydes and ketones, described above, both decrease the carbonyl carbon's oxidation state, and may be classified as reductions. As noted, they proceed by attack of a strong nucleophilic species at the electrophilic carbon. Other useful reductions of carbonyl compounds, either to alcohols or to hydrocarbons, may take place by different mechanisms.

For example, hydrogenation Pt, Pd, Ni or Ru catalysts , reaction with diborane, and reduction by lithium, sodium or potassium in hydroxylic or amine solvents have all been reported to convert carbonyl compounds into alcohols. However, the complex metal hydrides are generally preferred for such transformations because they give cleaner products in high yield. Aldehydes and ketones may also be reduced by hydride transfer from alkoxide salts. To learn about this method Click Here.

The reductive conversion of a carbonyl group to a methylene group requires complete removal of the oxygen, and is called deoxygenation. In the shorthand equation shown here the [H] symbol refers to unspecified reduction conditions which effect the desired change. Three very different methods of accomplishing this transformation will be described here.

Reaction of an aldehyde or ketone with excess hydrazine generates a hydrazone derivative , which on heating with base gives the corresponding hydrocarbon. A high-boiling hydroxylic solvent, such as diethylene glycol, is commonly used to achieve the temperatures needed. The following diagram shows how this reduction may be used to convert cyclopentanone to cyclopentane.

A second example, in which an aldehyde is similarly reduced to a methyl group, also illustrates again the use of an acetal protective group. The mechanism of this useful transformation involves tautomerization of the initially formed hydrazone to an azo isomer, and will be displayed on pressing the "Show Mechanism" button.

The strongly basic conditions used in this reaction preclude its application to base sensitive compounds. This alternative reduction involves heating a carbonyl compound with finely divided, amalgamated zinc.

The mercury alloyed with the zinc does not participate in the reaction, it serves only to provide a clean active metal surface. The first example below shows a common application of this reduction, the conversion of a Friedel-Crafts acylation product to an alkyl side-chain. The second example illustrates the lability of functional substituents alpha to the carbonyl group.

A possible mechanism for the Clemmensen reduction will be displayed by clicking the "Show Mechanism" button. In contrast to the previous two procedures, this method of carbonyl deoxygenation requires two separate steps. The asterisk next to the OH is because under basic conditions, OH becomes O - and therefore as a leaving group it is O 2- , which is extremely bad. Read at lest one time. In number 3, why is Cl considered the worst pi electron donator out of the three? Hi Chelsea — the electronegativity of Cl is actually less than that of O, but the major factor is orbital overlap.

The lone pairs of the Cl are in 3p orbitals whereas the carbonyl pi-bond is comprised of 2p orbitals. The size mismatch leads to poorer bonding overall. The situation is even worse with Br ; acyl bromides are very unstable. Note that if I put F instead of O it would still have poorer pi-donation, but in that case I really would say that it was because F is more electronegative than oxygen. Your email address will not be published.

Save my name, email, and website in this browser for the next time I comment. Notify me via e-mail if anyone answers my comment. This site uses Akismet to reduce spam. In this case, since this overlap is diminished, it turns out that thioesters are typically roughly as reactive towards nucleophilic attack as ketones.

Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Among thioester and ester which has more electrophilic carbonyl carbon? Ask Question. Asked 3 years, 10 months ago. Placing a highly electronegative grouping like CF3 adjacent to the carbonyl makes the carbonyl much more electrophilic, which makes it better able to stabilize negative charge.

On the other hand, carbonyls with electron-donating groups attached like amides do not stabilize negative charge nearly as well. This translates to lowered acidity of the alpha proton. These effects are additive, by the way. With this principle in mind, you should be able to identify rank the following dicarbonyl compounds on the basis of acidity. Ethene ethylene reacts well with electrophiles like Br2 and mCPBA, but when combined with a nucleophile like diethylamine, no reaction happens.

The reason for this lack of reactivity, just as it was for 6, is that the resulting carbanion that would result from nucleophilic attack is highly unstable. Replace a hydrogen with a carbonyl derivative, however, and suddenly the alkene becomes a lot more frisky.

Again, the reason is the ability of the electrophilic carbonyl group to stabilize the newly generated negative charge. The immediate product of conjugate addition is an enolate , which can then be quickly protonated, depending on reaction conditions. The key point is that the carbonyl stabilizes the intermediate of the reaction , which allows it to proceed. By knowing the relative donation ability of each substituent at the carbonyl e. Since nucleophilic addition is the rate-limiting step of this reaction , you will therefore know the relative rate of addition.

They come up in reactions again and again. By understanding their relative stability, you also understand their relative reactivity. My 3rd year organic chemistry teacher, Prof. Szarek, had us say the following out loud in class, as a group, several times a lesson, until we finally got this fact drilled into our heads:.